Power converting circuits as known in the art include ac-to-dc rectifier circuits, dc-to-dc converting circuits and ac-to-ac converting circuits (herein sometimes collectively referred to as "power conversion circuits," "power converting circuits," or "converters"). In each such circuit, one or more semiconductor switching devices are coupled between an input and an output of the power converting circuit. The input of the power converting circuit is coupled to a power source, such as a battery, the output of a rectifier circuit, or a three-phase ac source. The output of the power converting circuit is coupled to an electrical load to which the power provided by the power converting circuit is to be delivered. The switching devices of the power converting circuit are selectively controlled to switch on and off to condition the power received from the power source for delivery to the electrical load. For example, an ac-to-ac converting circuit may include a rectifier circuit for converting three-phase ac input into a dc voltage. The switching devices of a three-phase inverter circuit having its input connected to receive dc output of a rectifier circuit are connected and operated to provide a desired three-phase ac output.
The semiconductor switching devices utilized in power converting circuits can include thyristors, transistors, gate turn-off devices, MOSFETs, MCTs, or IGBTs. Power converting circuits typically include a snubber circuit connected across each semiconductor switching device. The snubber circuit operates to limit the rate of change of voltage across the switching device when the device is turned off, i.e., is opened, thereby reducing turn-off power dissipation losses within the switching device. Various types of snubber circuits have been proposed. Although existing snubber circuits serve to reduce losses from switching devices, they often produce a power loss themselves, thereby limiting their application to small power or low frequency circuits. When high frequency and/or high power is desired, loss within a snubber circuit can significantly reduce the power handling capacity of the converter.
To overcome active device switching losses, while enabling operation at higher switching frequencies, soft-switching converters have been developed. In general, there are two types of soft-switching (or resonant) converters: zero-voltage switching and zero-current switching. Zero-voltage switching involves switching an active device when there is zero-voltage thereacross. On the other hand, zero-current switching involves switching an active device when there is zero-current therethrough. Examples of soft-switching converters exist in the open literature, such as: U.S. Pat. Nos. 4,730,242; 4,864,483; and 5,038,267.
Despite developments in soft-switching converter technology, for many applications such converters remain too complex and/or costly, typically requiring the use of multiple active devices. Thus, there exists a need in the semiconductor art for an improved soft-switching, active snubber circuit (e.g., for application to the switching device(s) of a power converting circuit) that operates more efficiently than conventional snubber circuits. The present invention provides such a snubber circuit, which has both zero voltage turn-off switching and zero current turn-on switching.